A variety of vehicle suspension systems providing variable height and/or position control have been developed. U.S. Pat. No. 3,262,711 to Behles discloses a suspension leveling device that utilizes hydraulic/pneumatic spring elements to provide level control. Spring action to absorb bumps and jolts from the road is limited to the single action of the pneumatic portion of the springs. Although a spring element is provided at each corner of the vehicle, the elements work in tandem to position either axle. Accordingly, only limited fore and aft adjustment is available. Further, the system fails to provide any lateral (side-to-side) control. Thus, positive cornering control is simply not possible. Further, lateral vehicle stability, especially at highway speeds and in cross wind conditions, leaves much to be desired.
U.S. Pat. No. 4,625,993 assigned to Lotus discloses a vehicle suspension system including double acting hydraulic actuators at each wheel. These hydraulic actuators are mounted in parallel with the suspension spring assembly. This arrangement inherently provides relatively stiff ride characteristics since the hydraulic fluid is incompressible. The hydraulic actuators thus are designed to control vehicle heave, pitch and roll characteristics exclusively. The system includes a central computer, accelerometers mounted at each wheel and linear variable differential transformers mounted at each wheel. While this system is thus designed to provide some enhanced control characteristics, it hinders the ride characteristics, and in addition it is complicated and expensive to fit onto vehicles.
U.S. Pat. No. 4,625,994 assigned to Mitsubishi discloses a vehicle suspension apparatus including air spring chambers at each wheel. The air springs allow for a smoother, softer ride over generally small amplitude bumps, but there is no back-up spring action to absorb the larger amplitude bumps and jolts and to stabilize the suspension. Furthermore, this system is designed and implemented to work in a position control mode or a height control mode. The flexibility and utility of this system is limited, however, snce employment of these modes is mutually exclusive. As in the Lotus system, the design is highly complicated and expensive.
Another recently developed adaptive suspension substitutes pneumatic springs in the place of coil springs on a MacPherson strut front suspension. The pneumatic springs are thus the sole source of spring action, as in the Mitsubishi system. The pneumatic springs also provide a means to control vehicle height. In order to implement position or cornering control, this system includes means to change the shock absorber valving. The use of an onboard computer is required as are inputs from a steering wheel position sensor. Here again, this system is complicated and requires computer control in response to numerous vehicle inputs.
Yet another recently developed suspension system includes single air bags mounted on quarter elliptic composite leaf springs. The air bags act in parallel with the leaf springs. Thus for all road conditions, both the air bag and the leaf spring must flex. This inherently provides a stiffer and less responsive suspension system especially for the smaller amplitude bumps. This system is designed to be used on small trucks and is specifically intended to provide low loading heights. Thus, this prior art system provides load control only with little or no emphasis on ride quality.
A need, therefore, exists for an improved adaptive vehicle suspension system that has a major emphasis on ride quality, but also provides both automatic vehicle load and cornering control. Such a system would provide substantially improved ride and vehicle control characteristics, and yet be relatively economical to implement.